Piston-type internal combustion engines which are operated with freely triggerable or controllable gas exchange or cylinder valves can be operated without a throttle valve, even in Otto-engine modes of operation. Load control in such methods is achieved by various opening and closing instants of the cylinder (gas exchange) valves. To intensify the motion of the inflowing quantity of air or fresh gas, it is appropriate to generate a pronounced flow field on the intake side when the piston-type internal combustion engine is operated with low rpm and in a low load range. In addition, there is the problem that at high rpm the minimal flight times attainable via the actuator are too long for operation at low loads, i.e., low engine moments. The idea arises of solving the problem by adapting the inlet cross section.
From German published Patent Disclosure DE-A 196 10 468, a method for load-dependent control of the cylinder valves of a piston-type internal combustion engine is now known, in which during partial load operation the opening stroke is shortened, at least for the gas inlet valve of a cylinder, thus uncovering a smaller flow cross section. By shortening the opening stroke for partial load operation as compared to the opening stroke for the normal operating mode, a correspondingly reduced flow cross section is obtained at the valve inlet, so that a correspondingly lesser quantity of air or fresh gas is also aspirated. However, the lesser flow cross section in the inlet region means a higher flow velocity of the entering gas, resulting in a better mixing in the cylinder, which leads to better mixture preparation. In the known method, the desired stroke shortening is accomplished by suitable triggering or controlling of the electromagnetic actuators that are provided for actuating the cylinder valves. This is done in such a way that with the gas inlet valve closed, immediately after the holding current at the closing magnet is turned off and the motion of the cylinder valve in the opening direction ensues, the closing magnet is re-supplied with current, so that the gas exchange valve is moved back into the closing position after only a short stroke. In this method, operation is possible either with a partial stroke only, or with a full stroke only.
From German Patent Disclosure DE-A 30 24 109, corresponding to U.S. Pat. No. 4,555,543, issued Jun. 19, 1994, it is known to relax the opener spring, via a switching magnet, so much that the armature, in the closing position of the cylinder valve, contacts the pole face of the closing magnet even when the closing magnet is rendered currentless. This is intended to enable starting the electromagnetic actuators with less expenditure of energy, because when the engine is started current is first supplied to the closing magnet of a cylinder valve; then the opener spring is prestressed, and correspondingly, for the onset of motion when the closing magnet is made currentless, the full spring force of the opener spring is available for accelerating the armature and thus the cylinder valve. With this system, though, it is not possible to vary the stroke length of the cylinder valve.